Chimps produced antibodies against Ebola if the vaccine was squirted into their mouth (photo) or injected intravenously.

Matthias Schnell

Ebola vaccine for great apes shows promise, but ethical hurdles may block further research

Swallowing just a few drops of a new vaccine could protect against the deadly Ebola virus. The new immunization is not meant for humans, but chimpanzees and gorillas, for which Ebola is a devastating disease as well. Yet the vaccine may never reach these great apes. Further tests are all but impossible because of new ethical rules, the researchers charge.

Ebola is best known as a killer of people, but the virus also causes epidemics in wildlife. A 2006 study estimated that an outbreak in 2002 and 2003 in the Republic of the Congo claimed the lives of 5000 gorillas, but others have said the impact of the disease is hard to measure. Still, Ebola is a real and possibly growing threat to great apes, says epidemiologist Fabian Leendertz of the Robert Koch Institute in Berlin, who studies pathogens in nonhuman primates. 

To address this problem, Peter Walsh, a disease ecologist at the University of Cambridge in the United Kingdom, and other researchers inserted a protein that’s on the surface of the Ebola virus into an existing live rabies vaccine that has been used on hundreds of millions of baits around Europe to fight rabies in foxes. At the University of Louisiana in Lafayette’s New Iberia Research Center, they injected four chimpanzees with the vaccine and gave it orally to another six animals.

After 28 days, both groups of chimpanzees had high amounts of antibodies against Ebola in their blood. In the lab, these antibodies showed similar activity against Ebola as those found in rhesus macaques that had been vaccinated in a previous study and had survived a challenge with the Ebola virus. “Everything indicates that if we take this to Africa and put it in chimps, it is going to be protective,” Walsh says.

The immune response was comparable to that of the only other Ebola vaccine ever tested in chimpanzees, but it has two advantages. The other vaccine had to be injected, which means getting close enough to the apes to tranquilize them; this one can be put on a bait, and then left for the animals to find. And just one dose suffices, whereas three are needed of the injected vaccine. That has “massive advantages for the field vaccination of wild apes that are difficult to locate in dense forest and fear human approach,” the authors write today in Scientific Reports.

“This paper is a step forward,” Leendertz says. Still, there are a number of hurdles before the vaccine can be used in the wild, he cautions. Unlike foxes, great apes are very wary about baits. And there are many competing animals that might get to the bait first. Developing a vaccine is the easy part, adds William Karesh, a veterinarian at EcoHealth Alliance in New York City. “The hard part is designing a successful vaccination campaign.”

Walsh, who is currently investigating this issue with automated camera traps in the Republic of the Congo, admits that these are major problems. “There are Gambian pouched rats all over the forest, and the minute you put anything out, they eat it,” he says. “I have to find a way to get around that and that’s what I’m focusing on now.”

But new U.S. rules on research with chimpanzees are another hurdle, Walsh says. Further improvements on the vaccine, for instance to prevent it from losing its activity in the tropical heat, would require another round of testing on captive animals. And that looks all but impossible at the moment, he says.

Biomedical research on chimpanzees has been declining for years, and a new rule issued by the U.S. government in 2016 requires a permit under the Endangered Species Act. Although the rule still allows research on captive chimps if it benefits wild populations, the restrictions have made it too expensive to maintain chimpanzee groups for research, says Walsh, who cut his own vaccine study short when the rules took effect last September. Walsh has titled his paper “The Final (Oral Ebola) Vaccine Trial on Captive Chimpanzees?”

Such trials should only be done when all other questions about the vaccine’s feasibility have been addressed, Leendertz says. But even then, the animal sanctuaries where many retired research chimps end up are unlikely to condone the rare studies designed to help animals in the wild, he says, because it might tarnish their public image. “People always think that animal welfare and conservation are basically the same, but here they are really pitted against each other and that creates a problem.” And the scientific commission of the United Nations’s Great Apes Survival Partnership, that Leendertz is a part of, warned that the consequences of a vaccination program also need to be investigated. “Human communities will need to be consulted and public awareness campaigns developed to avoid the potential of such a program backfiring and resulting in the perception that bushmeat is “safe” to consume, leading to a rise to hunting activities," a statement read.